Nanoparticles having metal oxide compositions are increasingly being used in numerous emerging applications. Some of these include the use of magnetic nanoparticles (e.g., magnetite) in magnetic refrigeration or magnetic cooling circuits Ferrite-type nanoparticles, in particular, are being intensely studied for their use in the fields of biomedicine, optics, and electronics.
Current methods for the production of nanoscale ferrites and other oxide ceramics generally entail calcining a precursor (e.g., a carbonate) at a high temperature, and then mechanical milling the calcined product to reduce the particle size. The process is energy and time intensive, generally difficult to control, and often requires several repetitions of the process before a final product is obtained.
Chemical processes, such as precipitation and sol-gel techniques, are also known for the production of metal oxide nanoparticles. However, these processes are typically more expensive than mechanical milling, and also generally highly limited with respect to size or shape control of the resulting particles. Often, a chemical or physical reduction step is needed to convert a metal oxide precursor to a metal oxide product. In addition, these processes often require a mechanical milling step to break up agglomerates formed during the reduction process.
The microbial synthesis of metal oxide nanoparticles is known. See, for example, U.S. Pat. Nos. 6,444,453 and 7,060,473. However, there are significant problems in the microbial process as currently practiced. For example, there is the difficulty of obtaining pure nanoparticle product bereft of microbial matter. Therefore, numerous lysing or washing steps are often required. There is also the difficulty in controlling the particle size or the morphology of the nanoparticles.
In order to make microbial synthesis of metal oxide nanoparticles a more convenient and commercially viable method, the microbial process is in need of an improvement whereby substantially pure nanoparticle product bereft of microbial matter can be obtained upon precipitation from microbes. There is also a need for a microbial process whereby the particle size and/or morphology of the nanoparticles can be controlled during microbial synthesis of the nanoparticles.